The use of local interconnect (LI) for connecting poly gates and emitters to diffusion areas and for connecting N+ and P+ across field oxide regions allow an increased packing density for submicron VLSI circuits and a reduction in parasitic junction capacitance. See Titanium Nitride Local Interconnect Technology for VLSI by T. Tang, C. C. Wei, R. A. Haken, T. C. Holloway, L. R. Hite and T. G. W. Blake, IEEE Trans. Electron Devices, Vol. ED-34, 3 (1987) p. 682. TiN has been used as an LI. However, TiN is very difficult to etch without damaging a silicide layer on a substrate.
Both TiN and TiSi.sub.2 LI structures have been developed which are compatible with a self-aligned silicide, or salicide, process. See HPSAC-Silicided Amorphous Silicon Contact and Interconnect for VLSI by D. C. Chen, S. Wong, P. V. Voorde, P. Merchant, T. R. Cass, J. Amano, and K. Y. Chiu, IEDM Tech. Dig., (1984) p. 118 and New Device Interconnect Scheme for Submicron VLSI by S. Wong, D. Chen, P. Merchant, T. Cass, J. Amano, and K. Y. Chiu, IEEE Trans. Electron Devices, Vol. ED-34, 3 (1987) p. 587. The original process for TiSi.sub.2 LI begins with deposition of an amorphous silicon layer onto titanium. The Si layer is photographically defined and etched. Annealing in N.sub.2 causes the remaining Si to react with the underlying Ti to form TiSi.sub.2, while exposed Ti regions react with the ambient N.sub.2 to fore TiN. The TiN is then selectively removed with an H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 solution. In the original process, both contacts and LI are formed simultaneously. Because a high selectivity to Ti as between Ti and Si can be achieved by dry etching in a fluorine-based chemistry, this process has an etch advantage over the standard TiN LI process, which exhibits less selectivity to the salicided substrate. However, there are two major disadvantages of this process:. namely, thermal diffusion of substrate Si into the overlying titanium layer can lead to pitting of the substrate, and counterdoping between P+ and N+ regions may occur through the LI strap during both silicide formation and subsequent high temperature processing since phosphorous diffuses rapidly in TiSi.sub.2. The invention's new TiSi.sub.2 clad LI structure was developed to overcome these problems.